Rotational isomerization of 3-substituents in synthetic chlorophyll derivatives

Methyl pyropheophorbides- a possessing a (pseudo)planar substituent at the 3-position were prepared from naturally occurring chlorophyll- a . Some of the semisynthetic π-conjugates with the chlorin skeleton took two atropisomeric conformations for the sterically demanding 3-substituents, CONMe 2 , NHCOMe, C[CH C(CN) 2 ] C(CN) 2 , and Ph(2,3,4,5-Ph 4 ). Their rotational isomerization in a solution was analyzed by 1 H NMR and HPLC. Zinc complex of the 3-aryl-chlorin gave a large energy barrier for the rotation of the C3 C3 1 single bond (estimated Δ G ‡ =108 kJ mol −1 at 20 °C) and the atropisomerically pure conformers were separated at room temperature.